Piping systems of various types have been developed for transmitting fluids from one place to another. A challenge in using piping systems is connecting pipe sections and other components to avoid leakage at the joint between component parts or sections of pipe. Flanges have been developed to address this problem, and are now commonly used to connect sections of piping and to accommodate piping components such as valves, flow meters, branch connections and connections to tanks or pumps. Flanges are fabricated from a variety of materials including metals, thermoplastics, and fiber-reinforced plastic (FRP). All pipe flanges, whether metallic or plastic, are susceptible to two failure mechanisms: compromised strength and leakage. FRP flanges are much more flexible than metal flanges since they are fabricated from anisotropic materials and, as such, are more susceptible to bending and cracking forces which manifest as soon as the flange bolts are tightened, and increase thereafter when the piping system is subjected to internal pressure.
Because of the susceptibility of flanges to leak or fail, many piping standards recommend minimizing the number of flange connections. Despite the concerns and limitation of flanges, flanges are the only means for connecting piping to components such as valves, flow meters, pumps, instruments, and tanks and, as such, are very important in efficient operation of the system. Current standards for FRP flanges are based on metallic flange standards. This requires FRP flanges to match metal flange standards in bolt patterns and flange dimensions. Due to the lower modulus of elasticity of FRP, sealing is more difficult because of flange bending and the possibility of cracking when soft gaskets are used. Other code standards are similarly deficient regarding FRP flanges; for example, the fiberglass pipe design codes lack reliable design guidelines for flanges.
The problem of leakage at flange joints has been addressed by adapting various kinds of gaskets or seals that create a more or less static seal between adjacent flanges (or other components). For example, a commonly used gasket is the standard flat gasket seal shown in FIG. 1A. FIG. 1A shows a sectional view of two flanges 10 on pipes 11 with a flat gasket seal 15 between the two flanges 10. Also shown are the exteriors 12 and interiors 13 of the pipes 11. At the top of the flanges 10 are bolt holes 14. Soft flat gaskets are universally specified and are the standard for FRP flanges. Soft gaskets are generally made of rubber or soft elastomeric material that will compress to seal irregularities in the mating FRP surfaces. Such gaskets are positioned to cover the full face of the flange. Mating flanges are joined by bolting together at a torque calculated to properly seat the gaskets while limiting extrusion of the gasket from the interface. Soft gaskets normally require a low unit compression to achieve a leak-tight seal. However, the bolt loading must be adequate to prevent the flanges from bending under operating pressure of the piping system. Another commonly used gasket, shown in a sectional view in FIG. 1B, is a ring gasket 16.
Rubber gasket specifications are provided in ASTM D 1330 standard. If a gasket material is harder than the FRP flange material, the mating surfaces can be damaged possibly resulting in leakage. Hence, there is a need for soft gasket materials. PS 15-69 Voluntary Product Standard recommends a gasket material with durometer shore A or shore A2 hardness of 40 to 70 (70 being harder than 40). This standard, although originally intended for contact molded flanges, is widely used for the design of other types of FRP flanges.
The initial gasket preload is provided by the tensile load in the bolts, which is commonly known as the seating stress. Seating causes the gasket to deform and fill the irregularities on the flange surfaces to ensure contact over the sealing surface. As internal pressure is applied the gasket preload decreases and its ability to seal is reduced. The pressure load induces bending in the flange face and further reduces the sealing capability. If higher initial bolt loads are applied to overcome the pressure load on the flange, bending of the flange increases and tends to promote extrusion of the gasket.
To overcome some of the negative effects associated with soft flat gaskets, such as the effects of pressure and loss of contact pressure, O-ring gaskets or seals have been developed. FIG. 1C shows, in a sectional view, an example of an O-ring gasket 17. O-ring gaskets are effective in sealing all types of flanges and have been very successful when used on metal flanges. O-ring seals require a close tolerance groove 18 to be machined in one of the flange faces to retain the seal and provide controlled compression of the seal. The groove must be machined to a controlled depth to ensure adequate preload of the seal when the flanges are mated and bolts are tightened to the design load. FIG. 1C also shows a bolt 19 in place in bolt holes 14. FIG. 2 depicts one of the problems encountered with prior art devices, flange bending that results from the force of pressure (indicated by the arrows) resulting in a partial separation or gapping of gasket 15 from flanges 10.
While the standard O-ring design has proven to be very successful against leakage, the machined groove produces increased stress at the point of highest flange stress, namely, at the hub-flange interface. As a result, cracking of the flange hub has been observed when the piping system is subjected to pressure and/or bending stresses. Cracking of this type leads to leakage and eventual flange failure. FIG. 3A shows, in a sectional view, an example of horizontal cracks 20 in a flange 10. FIG. 3B shows, in a sectional view, an example of diagonal cracks 21 in a flange 10.
Additional problems arising with O-ring grooves relate to the requirement for a corrosion barrier. Corrosion barriers consist of a resin-rich layer of glass or polyester veil and two layers of chopped strand mat which protect the structural layers of pipe and flange from chemical attack. Corrosion barriers are placed on the face of each flange. When an O-ring groove is cut through the corrosion barrier the grooved area becomes susceptible to chemical attack. This requires replacement of the corrosion barrier which in turn changes the O-ring groove dimensions. To compensate for this change the groove dimensions must accommodate the barrier thickness. Such accommodations add to the cost of machining the groove and the overall cost of the flange. There remains a need for an improved O-ring design for use with FRP and thermoplastic flanges.